The trial heard how Forster had an “extreme right-wing” mindset and collected terrorist material.
He was arrested at his home in May 2022 by Met counter-terror police.
When asked for his phone, he claimed he had lost it at the pub - but it was found hidden under his mattress.
Detectives found an “appalling” library of abuse material, including more than 750 indecent pictures and videos of children aged as young as three.
Forster also had manuals describing how to make weapons and explosives.
Commander Dominic Murphy, head of the Met’s Counter Terrorism Command, said: “Forster built an appalling library of terrorist and child abuse material.
“His stockpile included the most serious types of child abuse images, and detailed bomb-making instructions. We also found texts and paraphernalia that highlighted his extreme right-wing mindset.
“It’s thanks to the thoroughness of counter terrorism investigators that Forster has been held to account both for the terrorism offences and child abuse crimes.”
Forster was convicted of three counts of collection of information of a kind likely to be useful to a person committing or preparing an act of terrorism, contrary to section 58 of the Terrorism Act 2000, and three counts of making indecent images of children (categories A, B and C), contrary to the Protection of Children Act 1978.
Alongside an additional year on licence, he will be subject to a ten-year terrorism notification order and a ten-year sexual harm prevention order.